Optical amplifier having a variable attenuator controlled based on input power

ABSTRACT

An optical amplifier is disclosed having substantially uniform spectral gain. The amplifier comprises a variable optical attenuator coupled between first and second segments of active optical fiber. The attenuation of the optical attenuator is adjusted in accordance with the optical power input to the amplifier to thereby obtain substantially flattened gain. Alternatively, the attenuator can be controlled based on the respective gains associated with the first and second segments of optical fiber. For example, the attenuator can be adjusted so that so that the sum of the two gains remains substantially constant, a condition that also yields flat spectral gain. Further, optical powers associated with first and second wavelengths of amplified stimulated emission (ASE) light output from the amplifier can be used to adjust the attenuation of the optical attenuator. In an additional example, received optical powers associated with each of the channels in a WDM system are monitored and the attenuators within each amplifier in the system are controlled so that the received powers are substantially equal.

BACKGROUND OF THE INVENTION

The present invention is directed toward optical amplifiers having asubstantially flat spectral gain.

Wavelength division multiplexing (WDM) has been explored as an approachfor increasing the capacity of existing fiber optic networks. In a WDMsystem, plural optical signal channels are carried over a single opticalfiber with each channel being assigned a particular wavelength. Suchsystems typically include a plurality of receivers, each detecting arespective channel by effectively filtering out the remaining channels.

Optical channels in a WDM system are frequently transmitted over silicabased optical fibers, which typically have relatively low loss atwavelengths within a range of 1525 nm to 1580 nm. WDM optical signalchannels at wavelengths within this low loss "window" can be transmittedover distances of approximately 50 km without significant attenuation.For distances beyond 50 km, however, optical amplifiers are required tocompensate for optical fiber loss.

Optical amplifiers have been developed which include an optical fiberdoped with erbium. The erbium-doped fiber is "pumped" with light at aselected wavelength, e.g., 980 nm, to provide amplification or gain atwavelengths within the low loss window of the optical fiber. However,erbium doped fiber amplifiers do not uniformly amplify light within thespectral region of 1525 to 1580 nm. For example, an optical channel at awavelength of 1540 nm, for example, is typically amplified 4 dB morethan an optical channel at a wavelength of 1555 nm. While such a largevariation in gain can be tolerated for a system with only one opticalamplifier, it cannot be tolerated for a system with plural opticalamplifiers or numerous, narrowly-spaced optical channels. In theseenvironments, much of the pump power supplies energy for amplifyinglight at the high gain wavelengths rather than amplifying the low gainwavelengths. As a result, low gain wavelengths suffer excessive noiseaccumulation after propagating through several amplifiers.

Accordingly, optical amplifiers providing substantially uniform spectralgain have been developed. In particular, optical amplifiers including anoptical filter provided between first and second stages of erbium dopedfiber are known to provide gain flatness. In these amplifiers, the firststage is operated in a high gain mode and supplies a low noise signal tothe second stage, while the second stage is operated in a high powermode. Although the second stage introduces more noise than the first,the overall noise output by the amplifier is low due to the low noisesignal of the first stage. The optical filter selectively attenuates thehigh gain wavelengths, while passing the low gain wavelengths, so thatthe gain is substantially equal for each wavelength output from thesecond stage.

These gain-flattening amplifiers are typically designed to receiveoptical signals at a particular power level. In the event the totalpower level of all optical signals input to the amplifier differs fromthe desired input level, the amplifier can no longer amplify eachwavelength with substantially the same amount of gain. Accordingly, theconventional gain-flattened amplifiers discussed above are unable toreceive input optical signals over a wide range of power levels whilemaintaining substantially uniform gain at each wavelength.

SUMMARY OF THE INVENTION

Consistent with an embodiment of the present invention, an opticalamplification device is provided, comprising a first segment of activeoptical fiber having a first end portion coupled to an opticalcommunication path carrying a plurality of optical signals, each at arespective one of a plurality of wavelengths, and a second end portion.The first segment of active optical fiber receives the plurality ofoptical signals through the first end portion and outputs the pluralityof optical signals through said second end portion. An opticalattenuator is also provided having an input port receiving the pluralityof optical signals coupled to the second end portion of the firstsegment of optical fiber. The optical attenuator further includes acontrol port that receives an attenuation control signal, and an outputport.

In addition, the optical amplification device comprises a second segmentof active optical fiber having a first end portion coupled to the outputport of the optical attenuator and a second end portion. The pluralityof optical signals propagate through the optical attenuator and aresupplied to the first end portion of the second segment of activeoptical fiber via the output port of said optical attenuator. Theplurality of optical signals are next output from the second segment ofactive optical fiber via the second end portion of the second segment ofactive optical fiber.

A control circuit is further provided which is configured to be coupledto the optical communication path. The control circuit senses an opticalpower of at least one of said plurality of optical signals, and outputsthe attenuation control signal in response to the sensed optical power.The optical attenuator, in turn, attenuates the plurality of opticalsignals in response to the attenuation control signal such that a powerassociated with each of said plurality of optical signals output fromthe second end portion of the second segment of active optical fiber issubstantially the same.

In accordance with an additional embodiment of the present invention,the attenuation of the optical attenuator is controlled in accordancewith the respective gains of the first and second segments of activeoptical fiber. For example, the attenuation is adjusted so that the sumof the gains of the two segments of active optical fiber remainsconstant.

In a further embodiment of the present invention, optical powersassociated with amplified stimulated emission light at a first andsecond wavelengths is compared, and the attenuation of the opticalattenuator is adjusted so that these optical powers are substantiallyequal.

Moreover, in accordance with an additional embodiment of the presentinvention, received power of each of a plurality of WDM signals ismeasured after propagation through a chain of amplifiers, each of whichincluding first and second segments of active optical fiber and anoptical attenuator coupled between the two. Based on the received power,the attenuation of the optical attenuator in each amplifier is adjustedso that the received power associated with each WDM signal issubstantially the same.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be apparent from the followingdetailed description of the presently preferred embodiments thereof,which description should be considered in conjunction with theaccompanying drawings in which:

FIG. 1 illustrates an optical amplifier in accordance with a firstembodiment of the present invention;

FIG. 2 illustrates an optical amplifier in accordance with a secondembodiment of the present invention;

FIG. 3 illustrates an optical amplifier in accordance with a thirdembodiment of the present invention;

FIG. 4 illustrates a service channel add/drop configuration inaccordance with an aspect of the present invention;

FIG. 5 illustrates an optical amplifier in accordance with a fourthembodiment of the present invention;

FIG. 6 illustrates an optical amplifier in accordance with a fifthembodiment of the present invention;

FIG. 7 illustrates an additional service channel add/drop configurationin accordance with an aspect of the present invention;

FIG. 8 illustrates an optical amplifier in accordance with a sixthembodiment of the present invention;

FIG. 9 illustrates an optical amplifier in accordance with a seventhembodiment of the present invention;

FIG. 10 illustrates an ASE sensor circuit in accordance with a furtheraspect of the present invention;

FIG. 11 illustrates an ASE sensor circuit in accordance with anadditional aspect of the present invention;

FIG. 12 illustrates a block level diagram of an optical communicationsystem in accordance with the present invention; and

FIG. 13 illustrates an additional ASE sensor circuit.

DETAILED DESCRIPTION

Turning to the drawings in which like reference characters indicate thesame or similar elements in each of the several views, FIG. 1illustrates an amplifier 100 in accordance with a first embodiment ofthe present invention. Optical amplifier 100 includes a known coupler102 having an input port 102-1 receiving a plurality of optical signals,each at a respective one of wavelengths λ₁ to λ_(n) typically within arange of 1500 to 1590 nm. The plurality of optical signals, which canconstitute WDM signals, are carried by an optical fiber 103. Coupler 102can constitute a conventional optical tap or splitter, which supplieseach of the plurality of optical signals to both outputs 102-2 and102-3. The power of optical signals at output 102-2 is typicallysignificantly more than the power of optical signals supplied fromoutput 102-3. For example, the power at output 102-3 can beapproximately 2% of the power fed to input 102-1, while the power atoutput 102-2 can be approximately 98% of the power supplied to input102-1 (neglecting coupler loss, for simplicity).

The optical signals output from coupler 102 are next supplied to a firstsegment of active optical fiber 104, which provides a first stage ofamplification. Active optical fiber 104 is typically doped with afluorescent material, such as erbium, and pumped with light at awavelength different than the amplified optical signals, e.g., 980 nm. Apump laser (not shown) is typically coupled to active optical fiber 104in a known manner to excite the fluorescent material. The pump light isof sufficient magnitude and the composition of active optical fiber 104is such that the optical signals output from coupler 102 are amplifiedwith high gain, but with relatively little noise.

The optical signals are next supplied to an input port 106-1 of filter106 via an isolator (not shown). Filter 106, commercially available fromJDS Fitel, for example, selectively attenuates certain optical signalwavelengths, e.g., the high gain wavelengths output from first segmentof active optical fiber 104, while permitting other wavelengths to passsubstantially unattenuated. The optical signals, some of which beingattenuated, next pass via filter output port 106-2 to input port 108-1of optical attenuator 108.

Optical attenuator 108 has an attenuation which can be variablycontrolled in accordance with an attenuation control signal supplied tocontrol port 108-2. Optical attenuator 108, which is commerciallyavailable from JDS Fitel and E-Tek, for example, attenuates each of theoptical signals by substantially the same amount, and as discussed ingreater detail, controllably attenuates the optical signals so thatamplifier 100 provides substantially uniform gain for each of theoptical signals.

The optical signals are then supplied via output port 108-3 ofattenuator 108 to a second segment of active optical fiber 110, whichprovides a second stage of amplification. Second segment of activeoptical fiber 110 is typically pumped with light from a laser (notshown) at a wavelength, e.g., 1480 nm, which is different than theoptical signal wavelengths λ₁ to λ_(n). In addition, second segment ofactive optical fiber 110 is pumped in such a manner and has anappropriate composition that yields a high power output to fiber 105.

As further shown in FIG. 1, optical signal output from port 102-3 ofcoupler 102 are supplied to a control circuit 107 including a knownphotodetector 112, which converts the optical signals to an electricalsignal, and an attenuator adjustment circuit 109 comprisinganalog-to-digital converter 114, interface circuit 116, memory circuit118 and digital-to-analog circuit 120. The electrical signal is suppliedto a known analog-to-digital converter circuit 114, which converts thereceived electrical signal, typically in analog form, to a digitalsignal. An interface circuit 116, including for example a decoder,couples the digital signal to a known memory circuit 118, such as anEPROM. Other circuitry may be provided between photodetector 112 andmemory circuit 118, as necessary, for example, voltage level adjustmentcircuits. Memory circuit 118 can constitute a look-up table, whichstores power values, as represented by the digital signal, andcorresponding attenuator adjustment values. Accordingly, in response tothe output from interface circuit 116, memory circuit 118 outputs anassociated attenuator adjustment value corresponding to a substantiallyflat amplifier gain spectrum. The attenuator adjustment value issupplied to digital-to-analog converter circuit 120, which, in turn,feeds an attenuation control signal to control port 108-2 of attenuator108, to appropriately adjust the attenuation thereof. Other circuitrymay be provided between memory circuit 118 and attenuator 108, asnecessary, e.g., for voltage level adjustment etc.

Thus, variations in input power to amplifier 100 at input port 102-1 ofcoupler 102 can be offset by corresponding changes in the attenuation ofoptical attenuator 108 so that optical amplifier 100 maintains asubstantially uniform gain spectrum.

FIG. 2 illustrates an amplifier 200 in accordance with an additionalembodiment of the present invention. Optical amplifier 200 is similar toamplifier 100 shown in FIG. 1, with the exception that attenuatoradjustment circuit 109 comprises a conventional microprocessor orcentral processing unit (CPU) 202, which receives the electrical signaloutput from photodetector 112 and calculates an appropriate attenuatoradjustment value required for amplifier gain flatness in responsethereto. CPU 202 further outputs an attenuation control signal inaccordance with the attenuator adjustment value so that amplifier 200maintains substantially uniform spectral gain.

FIG. 3 illustrates amplifier 300 in accordance with an alternativeembodiment of the present invention. Amplifier 300 is similar toamplifier 100 but attenuator adjustment circuit 109 comprises acomparator circuit 302 and predetermined electrical signal source orvoltage reference 304. In effect, attenuator adjustment circuit 109compares a power associated with the plurality of optical signals inputto coupler 102 with a predetermined power value, as represented by anappropriate voltage output from voltage reference 304, and outputs theattenuation control signal in response to this comparison.Alternatively, a suitable comparator circuit 302 could be coupled tointerface circuit 116 and a memory circuit, such as memory circuit 118or a register, storing the predetermined power value. In which case, thecomparator circuit would output the attenuation control signal inresponse to a comparison of the input power value and the predeterminedpower value stored in the memory circuit. The attenuation control signaloutput from comparator circuit 302 adjusts the attenuation of attenuator108 so that amplifier 300 has substantially flat spectral gain.

FIG. 4 illustrates a feature of the present invention, whereby a serviceor monitoring channel signals, having a wavelength typically lyingoutside the range of wavelengths (e.g., 1500 nm-1590 nm) of opticalsignals input to coupler 102, can be inserted and extracted fromamplifiers 100, 200, and 300, as discussed, for example, in U.S. Pat.No. 5,532,864, incorporated by reference herein. Filter 106, reflectsthe received service channel signals, typically having a wavelength of1625-1650 nm, to service channel receiver 402, and directs the servicechannel signal emitted by service channel transmitter 404 to input port108-1 of attenuator 108. The service channel add/drop configurationshown in FIG. 4 can also be incorporated into the amplifier shown inFIGS. 5, 8, 9 and 12, discussed in greater detail below. It is notedthat filter 106 can serve both purposes of adding/dropping the servicechannel, as well as selectively attenuating the high gain wavelengthi.e., for gain flattening. Alternatively, separate filters can beprovided for service channel add/drop and gain flattening, respectively.

FIG. 5 illustrates an alternative amplifier 500 in accordance with thepresent invention similar to amplifier 300 shown in FIG. 3. Amplifier500, however, further comprises a dispersion compensating element 502coupled between output port 108-2 of optical attenuator 502 and secondsegment of active optical fiber 110. Dispersion compensating element 502provides dispersion compensation for the optical signals output fromattenuator 108, and can include either a known dispersion compensatingfiber (DCF) or dispersion compensating Bragg grating. It is noted thatdispersion compensating element can be provided at any appropriatelocation within any one of the embodiments of the present invention,e.g., amplifiers 100, 200, 300, and the amplifiers shown in FIGS. 6, 8and 9.

FIG. 6 illustrates amplifier 600 in accordance with a further embodimentof the present invention. Amplifier 600 is similar to amplifier 300discussed above, but includes an additional filter 606. In thisembodiment, filter 106 typically attenuates one group of wavelengths,while filter 606 attenuates another group in order to provide gainflattening. Alternatively, as further shown in FIG. 7, filter 106 can beused to direct first service channel signals to service channel receiver402, while filter 606 can be used to couple second service channelsignals emitted by service channel transmitter 404 to second segment ofactive optical fiber 110 for output from amplifier 600. An additionalfilter can also be provided in FIG. 7 to provide spectral filtering ofthe high gain wavelengths to obtain flattened gain. Alternatively,filters 106 and 606 can be provided which both selectively attenuate thehigh gain wavelengths and perform the service channel add or drop.

The embodiment shown in FIG. 7 may be advantageous in having reducedcross-talk between the added and dropped service channel signalscompared to the add/drop configuration shown in FIG. 4. In particular,the add/drop configuration shown in FIG. 4 includes a single filter 106for both adding and dropping the service channel signals. Typically,filter 106, however, is not entirely reflective at the service channelwavelength. Accordingly, a portion of the second service channel signalsemitted by service channel transmitter 404 in FIG. 4 can pass throughfilter 106 to service channel receiver 402, thereby resulting incross-talk or interference between the received service channel signaland the portion of the service channel signal emitted by service channeltransmitter 404.

In FIG. 7, the service channel signals are dropped and added withseparate filters. Accordingly, any portion of the service channelsignals emitted by service channel transmitter 404 that propagatestoward attenuator 108, and not to second segment of active optical fiber110, as intended, are significantly attenuated by attenuator 108,thereby effectively eliminating any cross-talk at receiver 402. Theadd/drop configuration shown in FIG. 7 may be incorporated into all theamplifiers discussed above, as well as those shown in FIGS. 8, 9 and 12below.

FIG. 8 illustrates amplifier 800 in accordance with another embodimentof the present invention. In FIG. 8, the gains of first and secondsegments of active optical fiber 104 and 110 are determined, andadjusted by varying the attenuation of optical attenuator 108, so thatthe sum (or difference) of the two remains at a substantially constantvalue corresponding to a uniform spectral gain. For example, coupler 802taps a portion of the power of the optical signals input to firstsegment of active optical fiber 104 to photodetector 806, while coupler804 taps a portion of the power of the optical signals output from fiber104 to photodetector 808. Photodetectors 806 and 808, in turn, outputfirst and second electrical signals to a first gain monitoring circuit810. Appropriate processing of the received first and second electricalsignals is performed within gain monitoring circuit 810, and a knowndividing circuit, provided in gain monitoring circuit 810, divides afirst power value corresponding to the power output from fiber 104 by asecond power value corresponding to the power input to fiber 104. Theresulting quotient is the gain associated with first segment of activeoptical fiber 104. Gain monitoring circuit 810 then outputs to aregulator circuit, such as add and compare circuit 812, a first gainsignal in accordance with the gain associated with fiber 104.

As further shown in FIG. 8, coupler 820 taps a portion of the power ofthe optical signals input to second segment of active optical fiber 110,and coupler 822 taps a portion of the power of the optical signal outputfrom fiber 110. Photodetectors 818 and 816 respectively receive theoptical outputs of couplers 820 and 822, and generate correspondingthird and fourth electrical signals in response thereto. The third andfourth electrical signals are fed to a second gain monitoring circuit814, which appropriately processes these signals to obtain third andfourth power values corresponding to the power input and output fromfiber 110. A known dividing circuit within gain monitoring circuit 814divides the fourth power value by the third power value. The resultingquotient thus corresponds to the gain associated with second segment ofactive optical fiber 110. Based on this quotient, a second gain signalis output to add and compare circuit 812.

After receiving the first and second gain signals, add and comparecircuit 812 typically adds the two signals and compares the sum with apredetermined sum corresponding to a substantially flat gain conditionof the amplifier. As a result of the comparison, an attenuation controlsignal is supplied to attenuator 108 so that the sum of the first andsecond gain signals is adjusted to substantially equal the predeterminedsum. At which point, amplifier 800 has substantially uniform gain.

FIG. 9 illustrates a further embodiment of the present invention inwhich attenuator 108 is controlled in accordance with power levels ofamplified stimulated emission (ASE) light output from amplifier 900.Amplifier 900 comprises, for example, a coupler 902 that taps opticalsignals output from second segment of optical fiber 110. The opticalsignals are fed to ASE sensor circuit 904, which, in turn, detects theoptical power associated with first and second wavelengths of the ASElight. ASE sensor circuit then outputs an attenuation control signal tocontrol port 108-2 of attenuator 108 to adjust the attenuation thereofso that the optical powers associated with the two wavelengths of ASElight are substantially the same. At this point, amplifier 900 hassubstantially uniform spectral gain.

FIGS. 10 and 11 illustrate examples of ASE sensor circuit 904 comprisingdifferent optical demultiplexers. In the first example shown in FIG. 10,ASE sensor circuit 904 includes an optical demultiplexer comprisingfirst 1002 and second 1004 filters, commercially available from JDSFitel, for example, which receive light from coupler 902 and reflect acorresponding one of the ASE wavelengths, while transmitting otherwavelengths. Photodetectors 1006 and 1008, respectively coupled tofilters 1002 and 1004, output electrical signals in response to thereceived ASE light to comparator circuit 1010. Comparator circuit 1010,in turn, supplies the attenuation control signal to attenuator 108 tosubstantially equalize the power associated with the first and secondASE wavelengths, thereby flattening the gain of amplifier 900.

In FIG. 11, ASE sensor circuit 904 includes an optical demultiplexercomprising splitter 1102, couplers 1104 and 1110 and in-fiber Bragggratings 1106 and 1108. Optical splitter 1102 outputs light tapped fromcoupler 902 through outputs 1102-1 and 1102-2. A portion of the tappedlight supplied through port 1102-1 passes through a first coupler 1104to in-fiber Bragg grating 1106. In-fiber Bragg grating 1106 isconfigured to reflect light primarily at the first ASE wavelength, forexample. Accordingly, the first ASE wavelength light is reflected backto coupler 1104, which directs the light to photodetector 1006. In asimilar fashion, another portion of the tapped light from coupler 902 isfed to in-fiber Bragg grating 1108 via coupler 1110 and output 1102-2 ofsplitter 1102. In fiber Bragg grating 1108 typically reflects asubstantial portion of the ASE light at the second wavelength. Thus, theASE second wavelength light is reflected back to coupler 110, whichdirects the light to photodetector 1008. Photodetectors 1006 and 1008,as discussed above, output electrical signals to comparator circuit 1010in response to the received ASE light. Based on these electricalsignals, comparator circuit 1010 next outputs an appropriate attenuationcontrol signal to adjust the attenuation of attenuator 108 so that thereceived power of the ASE light at the first and second wavelength issubstantially the same. In which case, amplifier 900 has substantiallyflattened gain.

Although optical demultiplexers comprising in-fiber Bragg gratings andfilters are described above, arrayed waveguide gratings (AWGs) or othersuitable optical demultiplexers can be incorporated into ASE sensorcircuit 904. For example, as seen in FIG. 13, AWG 1310 outputs the ASElight at the first and second wavelengths through outputs 1310-1 and1310-2, respectively. The ASE light is supplied to photodetectors 1006and 1008, which operate in conjunction with comparator circuit 1010 in amanner similar to that described above to output an attenuation controlsignal to attenuator 108.

FIG. 12 illustrates an alternative embodiment of the present inventionin which the attenuators included in a chain of amplifiers are adjustedsubstantially simultaneously so that the power associated with eachoptical signal output from the chain is substantially the same. Inparticular, FIG. 12 illustrates a WDM system 1200 comprising a pluralityof transmitters Tr₁ to Tr_(n) (1202-1 to 1202-n) each of which emittingone of a plurality of optical signals. Each of the plurality of opticalsignals are at a respective one of a plurality of wavelengths. Theoptical signals are output to and combined, using a conventional WDMmultiplexer 1204, onto an optical communication path 1203, comprising,for example, an optical fiber. A chain of optical amplifiers 1206-1 to1206-5 are coupled in series along optical communication path 1203. Theoptical amplifiers can have a structure similar to that of any one ofoptical amplifiers discussed above, including an optical attenuatorcoupled between first and second segments of active optical fiber. A WDMdemultiplexer 1208 is coupled to optical communication path 1203 at theend of the amplifier chain. Each of the outputs of WDM demultiplexer1208 are coupled to a respective one of receivers 1210-1 to 1210-n,which convert the optical signals to corresponding electrical signals.Received power modules 1212-1 to 1212-n sense these electrical signalsand determine the received optical power and/or signal to noise ratioassociated with each optical signal. The received power modules supplypower level signals corresponding to the received optical powers tomonitor circuit 1214, which determines whether the received power levelsare substantially equal. If not, monitor circuit 1214 outputs anadjustment signal to tilt control circuits 1216-1 to 1216-5. In responseto the adjustment signal, each of tilt control circuits 1216-1 to 1216-5outputs a corresponding attenuation control signal to the attenuators inamplifiers 1206-1 to 1206-5, thereby adjusting the output powers of theoptical signals supplied from each of these amplifiers. Received powermodules, in turn, detect the new optical power levels and supply newpower level signals to monitor circuit 1214. Monitor circuit 1214typically continues to output adjustment signals to tilt controlcircuits 1216-1 to 1216-5, thereby maintaining substantially equal powerlevels for each optical signal.

While the foregoing invention has been described in terms of theembodiments discussed above, numerous variations are possible.Accordingly, modifications and changes such as those suggested above,but not limited thereto, are considered to be within the scope of thefollowing claims.

What is claimed is:
 1. An optical amplifier, comprising:an opticalcoupler having an input port, and first and second output ports, saidinput port being configured to be coupled to an optical communicationpath carrying a plurality of optical signals, each at a respectivewavelength, said optical communication path having a first length; afirst stage having a first segment of active optical fiber having afirst end portion coupled to said first output port of said opticalcoupler, and a second end portion, said first segment of active opticalfiber receiving said plurality of optical signals through said first endportion and outputting said plurality of optical signals through saidsecond end portion; an optical attenuator having an input port coupledto said second end portion of said first segment of optical fiber, saidoptical attenuator having a control port that receives an attenuationcontrol signal, and an output port, said input port of said opticalattenuator receiving said plurality of optical signals, a firstintermediate optical path coupling said input port of said opticalattenuator to said second end portion of said first segment of opticalfiber; a second stage having a second segment of active optical fiberhaving a first end portion coupled to said output port of said opticalattenuator and a second end portion, said plurality of optical signalspropagating through said optical attenuator and being supplied to saidfirst end portion of said second segment of active optical fiber viasaid output port of said optical attenuator, said plurality of opticalsignals being output from said second segment of active optical fibervia said second end portion of said second segment of active opticalfiber, a second intermediate optical path coupling said output port ofsaid optical attenuator to said first end portion of said second segmentof active optical fiber, lengths associated with said first and secondintermediate optical paths being less than a length of said opticalcommunication path; a photodetector coupled to said second output portof said coupler, said photodetector sensing a portion of said pluralityof optical signals and generating an electrical signal in responsethereto; and an attenuator adjustment circuit coupled to saidphotodetector and said control port of said optical attenuator, saidattenuator adjustment circuit outputting said attenuation control signalin response to said electrical signal, said optical attenuatorattenuating said plurality of optical signals in response to saidattenuation control signal such that a power associated with each ofsaid plurality of optical signals output from said second end portion ofsaid second segment of active optical fiber is substantially the same.2. An optical amplification device in accordance with claim 1, whereinsaid attenuator adjustment circuit comprises:a memory circuit coupled tosaid photodetector, said memory circuit outputting an attenuatoradjustment value in response to said electrical signal, said attenuatoradjustment circuit outputting said attenuation control signal inaccordance with said attenuator adjustment value.
 3. An opticalamplification device in accordance with claim 1, wherein said attenuatoradjustment circuit comprises:a comparator circuit having a first inputcoupled to said photodetector, to thereby receive said electricalsignal, and a second input; and a predetermined electrical signalsource, said predetermined electrical signal source supplying apredetermined electrical signal to said second input of said comparator,said comparator generating said attenuation control signal based on saidelectrical signal and said predetermined electrical signal.
 4. Anoptical amplification device in accordance with claim 1, furthercomprising an optical filter having an input port coupled to said secondend portion of said first segment of active optical fiber and an outputport coupled to said input port of said optical attenuator.
 5. Anoptical amplification device in accordance with claim 4, wherein saidoptical filter comprises an additional input port and an additionaloutput port, said optical amplification device further comprising:aservice channel transmitter coupled to said additional input port ofsaid optical filter, said service channel transmitter supplying firstoptical service signals at a wavelength different than said plurality ofoptical signals to said additional input port of said optical filter,said first optical service signals being output through said output portof said optical filter to said input port of said optical attenuator;and a service channel receiver coupled to said additional output port ofsaid optical filter, said service channel receiver sensing secondoptical service signals output from said additional output port of saidoptical filter.
 6. An optical amplification device in accordance withclaim 1, further comprising:a first optical filter having an input portcoupled to said second end portion of said first segment of activeoptical fiber and an output port coupled to said input port of saidoptical attenuator; and a second optical filter having an input portcoupled to said output port of said optical attenuator and an outputport coupled to said fist end portion of said second segment of activeoptical fiber.
 7. An optical amplification device in accordance withclaim 6, wherein said first optical filter includes an additional outputport and said second optical filter includes an additional input port,said optical amplification device further comprising:a service channelreceiver coupled to said additional output port of said first opticalfilter, said service channel receiver sensing first optical servicesignals output from said additional output port of said first opticalfilter, said first optical service signals being at a wavelengthdifferent than said plurality of optical signals; and a service channeltransmitter coupled to said additional input port of said second opticalfilter, said first optical service signals being output through saidoutput port of said second optical filter to said first end portion ofsaid second segment of active optical fiber.
 8. An optical amplificationdevice in accordance with claim 1, further comprising:an optical filtercoupled between said first and second segments of active optical fiber;and a dispersion compensating element provided between said first andsecond segments of active optical fiber.
 9. An optical amplificationdevice in accordance with claim 8, wherein said dispersion compensatingelement comprises dispersion compensating fiber.
 10. An opticalamplification device in accordance with claim 8, wherein said dispersioncompensating element comprises a dispersion compensating Bragg grating.11. A method for controlling an optical amplifier, said opticalamplifier being configured to be coupled to an optical communicationpath having a first length, said optical amplifier comprising first andsecond segments of active optical fiber coupled in series by anintermediate optical path having a second length less than said firstlength, and an optical attenuator coupled between said first and secondsegments of active optical fiber, said method comprising the stepsof:supplying a plurality of optical signals to said first segment ofactive optical fiber, each of said plurality of optical signals being ata respective one of a plurality of wavelengths; measuring an inputoptical power associated with at least one of said plurality of opticalsignals supplied to said first segment of active optical fiber;outputting said plurality of optical signals via said optical attenuatorand said second segment of active optical fiber; adjusting anattenuation of said optical attenuator based on said input optical powersuch that an output power associated with each of said plurality ofoptical signals output from said optical amplifier is substantially thesame.
 12. A method for controlling an optical amplifier, said opticalamplifier being configured to be coupled to an optical communicationpath having a first length, said optical amplifier comprising first andsecond segments of active optical fiber coupled in series by anintermediate optical path having second length less than said firstlength, and an optical attenuator coupled between said first and secondsegments of active optical fiber, said method comprising the stepsof:supplying a plurality of optical signals to said first segment ofactive optical fiber, each of said plurality of optical signals being ata respective one of a plurality of wavelengths; measuring an inputoptical power associated with at least one of said plurality of opticalsignals supplied to said first segment of active optical fiber;outputting said plurality of optical signals via said optical attenuatorand said second segment of active optical fiber; obtaining apredetermined attenuation value stored in a memory corresponding to saidinput optical power; adjusting an attenuation of said optical attenuatorbased on said predetermined attenuation value such that an output powerassociated with each of said plurality of optical signals output fromsaid optical amplifier is substantially the same.
 13. A method forcontrolling an optical amplifier, said optical amplifier beingconfigured to be coupled to an optical communication path having a firstlength, said optical amplifier comprising first and second segments ofactive optical fiber coupled in series by an intermediate optical pathhaving a second length less than said first length, and an opticalattenuator coupled between said first and second segments of activeoptical fiber, said method comprising the steps of:supplying a pluralityof optical signals to said first segment of active optical fiber, eachof said plurality of optical signals being at a respective one of aplurality of wavelengths; measuring an input optical power associatedwith at least one of said plurality of optical signals supplied to saidfirst segment of active optical fiber; comparing said input opticalpower with a predetermined power value; outputting said plurality ofoptical signals via said optical attenuator and said second segment ofactive optical fiber; adjusting an attenuation of said opticalattenuator based on said comparison such that an output power associatedwith each of said plurality of optical signals output from said opticalamplifier is substantially the same.
 14. An optical amplifiercomprising:a first segment of active optical fiber having a first endportion coupled to an optical communication path having a first length,said optical communication path carrying a plurality of optical signals,each at a respective one of a plurality of wavelengths, and a second endportion, said first segment of active optical fiber receiving saidplurality of optical signals through said first end portion andoutputting said plurality of optical signals through said second endportion; an intermediate optical path coupled to said second end portionof said first segment of active optical fiber, said intermediate opticalpath having a second length less than said first length; an opticalattenuator having an input port coupled to said intermediate opticalpath, said optical attenuator having a control port that receives anattenuation control signal, and an output port, said input port of saidoptical attenuator receiving said plurality of optical signals; a secondsegment of active optical fiber having a first end portion coupled tosaid output port of said optical attenuator and a second end portion,said plurality of optical signals propagating through said opticalattenuator and being supplied to said first end portion of said secondsegment of active optical fiber via said output port of said opticalattenuator, said plurality of optical signals being output from saidsecond segment of active optical fiber via said second end portion ofsaid second segment of active optical fiber; a first optical filterhaving an input port coupled to said second end portion of said firstsegment of active optical fiber and an output port coupled to said inputport of said optical attenuator; and a second optical filter having aninput port coupled to said output port of said optical attenuator and anoutput port coupled to said fist end portion of said second segment ofactive optical fiber.
 15. An optical amplification device in accordancewith claim 14, wherein said first optical filter includes an additionaloutput port and said second optical filter includes an additional inputport, said optical amplification device further comprising:a servicechannel receiver coupled to said additional output port of said firstoptical filter, said service channel receiver sensing first opticalservice signals output from said additional output port of said firstoptical filter, said first optical service signals being at a wavelengthdifferent than said plurality of optical signals; and a service channeltransmitter coupled to said additional input port of said second opticalfilter, said first optical service signals being output through saidoutput port of said second optical filter to said first end portion ofsaid second segment of active optical fiber.